Getting the best performance from resin encapsulants

In this article on resin potting and encapsulation, Electrolube’s resins expert, Alistair Little, expands on a Q&A guide format with some advice on selection and application.

There are a number of different factors that influence the protection afforded by potting compounds. The act of encapsulating a component or PCB means that it is surrounded by a layer of resin, which completely seals a component or an entire PCB from the environment in which it operates.

A two-part resin, when mixed, starts a chemical reaction that results in the resin becoming fully polymerised to provide a homogenous layer. The polymerisation reaction creates a three-dimensional structure, which provides a barrier against chemical attack and high humidity, physical shocks and the potentially destructive effects of thermal cycling. So, let us refer to our questions.

What typical applications use epoxy resins?

Epoxies are typically used where extremes of temperature and chemically aggressive environments are encountered. Under-hood applications are common, and epoxies are often used to protect the huge variety of sensors, digital electronics and connectors that abound beneath the hood of a modern automobile — not just from high temperatures, but also from volatile fuels and lubricants.

Even as we switch from the internal combustion engine to the electric motor, the chemical environment might change, but the requirements for chemically resistant potting resins capable of tolerating a wide temperature range and resisting chemical attack will remain.

Their excellent adhesion to a wide range of substrates means that epoxies are used to ensure that the electronics are also very effectively sealed against the external environment and are perfectly adapted for use in equipment destined for deployment in ATEX and other hazardous areas.

What typical applications use silicones?

Silicones are used where extremes of temperatures are to be expected, or the components are temperature sensitive and would not survive the exothermic reactions that occur when two-part epoxies and polyurethanes are mixed and cured; this is particularly so when a large volume of material is needed for pouring into a single unit. Silicones are also suitable for applications where a high degree of flexibility is required, such as on flexible PCBs, and, unlike epoxies, are easy to remove if circuit modifications or repairs are required.

Silicone resins have the broadest continuous operating temperature range of any of the available resin chemistries, and so are a natural choice for both high- and low-temperature applications. They maintain their flexibility over this temperature range with very little sign of degradation over time. Due to their high flexibility, they place very low stresses on delicate components, particularly those with weak and fragile connecting legs.

There is, however, a downside to silicones, particularly the effectiveness of their adhesion to certain substrates. Moreover, their chemical resistance is not as good as that provided by an epoxy resin. Another category of resins — polyurethanes — would be the better choice for applications operating in the -30 to +120°C region as these offer similar levels of flexibility and better adhesion to many substrates.

What key ‘pain points’ are associated with resin selection?

Resin selection is the art of compromise; it is deciding which characteristic or property of the resin is more important to your application compared with those outlined above. Often the main areas of potential problems lie in deciding what are the realistic maximum and minimum requirements compared, for instance, to the design limits, which are likely to include large safety margins.

Viscosity is another property that must be considered. Where, normally, the lowest mixed viscosity resin is desired to promote excellent flow and coverage, there are thixotropic resins that behave somewhat differently in that their viscosity increases rapidly after mixing. This apparent change should not be confused with curing; the resin is still able to flow and has a useable life after it has stopped moving prior to reaching a final cure.

How do I overcome these ‘pain points’?

In most cases, many of the potential sticking points come down to the design brief and discussion between the designers and engineers as to what is feasible. If the high and low temperatures are only needed for a short period of time on an infrequent basis, and the normal operating temperature range is more modest, then this often opens up the choice to a much wider range of resins.

Similarly, when considering chemical resistance, determine whether the resin is actually the primary point of exposure. For example, if an LED is potted with an optically clear resin, but then a plastic cover with gasket is placed over the top of it, then the level of protection that the resin needs to provide is significantly reduced as, although it delivers the primary electrical insulation layer, it is only providing a secondary barrier against the environment.

To pot or not to pot? Why do we pot?

Naturally we would recommend that all electronic components, boards and units are either coated or potted. This is to extend the life of the finished unit and to protect the components against the environment. The level of protection required is dependent upon the environment to which the finished unit may become exposed. It might be indoors in a domestic setting, where a light layer of dust might be expected. Contrast this with a unit submerged in a garden pond for four months of the year, or one that is located in a hazardous industrial environment. Some units may potentially be used undersea for 20+ years or positioned on an aircraft subject to short haul commuter routes (with regular temperature and pressure variations). Other units may be exposed to the vacuum, extreme low temperatures and ionising radiation hazards of outer space; the variations really are endless.

Under less extreme conditions, you might ask if a resin is still the best option or if a coating should be considered as an alternative. This is an interesting point as the protection provided by a coating offers a number of advantages over the application of a resin. However, as always, the choice will depend upon what level of protection the designer requires. In addition, if you need to protect your intellectual property and avoid the underlying circuit being copied, then a resin will not only provide excellent protection due to its toughness, chemical resistance and adhesion to the substrate and components, but its opacity will ensure that the circuit detail is visually obscured.

If you have any questions, or would like more information about potting resins, their selection, handling and applications, contact our Technical Support Team and they will be more than happy to answer your queries.

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